In the wireless transmission of information signals, extinction of signal constituents may arise due to multiple-path reception. It is known to reduce such disturbances by reconstructing the signal with appropriate signal processing in the receiver and/or by selection of transmission frequencies less susceptible to multi-path contamination.
Concerning stationary receivers, it is only necessary to perform an adjustment of the receiver to the respective reception condition once, for example, after energizing the receiver. However, in receivers which are used in motor vehicles, a continuous adjustment to the fast-changing receiving conditions is necessary. This can be achieved by, for example, periodically transmitting a test sequence and evaluating it via correlation processes in the receiver. The correlation results determine the adjustment (corrective) values for an (adaptive) filter which is to be switched into the signal path. Such methods are described in the following papers, for example: G. D. Forney, Jr. "Training adaptive linear filters", U.S. Pat. No. 3,723,911 Mar. 27, 1973; and K. H. Mueller and D. A. Spaulding, "Cyclic equalization - a new rapidly converging equalization technique for synchronous data communication", Bell System. J., pp 369-406, February 1975. The adaptive methods which are described in these papers require a very significant technical effort, which may be accomplished, for example, with a calculator (signal processor) having appropriate capacity.
Typically this task is performed with a filter arrangement, including a shift register and a calculator, which can be continuously adjusted to changing reception conditions. The information "to be filtered" consists of, for example, a sequence of digital coded values each of 8 bits and is constantly fed to the shift register. The shift register is so proportioned that, for example, at least 1024 values each of 8 bits can be stored. Using this long length of, for example, 1024 values, a new output value is calculated according to predetermined mathematical laws for every single value by means of a calculator, for example, a signal processor. Each output value determined by the calculator depends on the 1024 different values. The mathematical law implemented is maintained constant for each output value generated for this length of information and possibly for considerably longer lengths of, for example, several thousand values. At the end of each sequence of input values of, for example, 1024 values, or at the end of even longer sequences of, for example, several blocks of 1024 bits each, the `creation rule` which has been assigned to the filter for the calculation of the new output values may be altered.
The process requires a large amount of computing effort for each starting value, for example, 1024 8-bit multiplications plus summation of the respective products. Calculations for successive output values are effectively carried out serially including the single starting value. The corresponding basic values for successive calculations are made available through intermediate rotations of the shift register. In the selected example the time available for the calculation of the longer lengths (series) of data amounts to a few milliseconds, i.e. about 1000 output values have to be calculated in about one millisecond, meaning about 1000 multiplications (per output value) per micro-second. This cannot be achieved even with the best available processors. Nor can such a data sequence be read out from a shift register in the allotted time. As the process time for a multiplication amounts to, for example, 25 ns, a correspondingly large number of processors must be operated in parallel, in order to solve the task, which can be ruled out because of the costs and other practical reasons relating to technical devices for consumers.
It is the object of the invention to provide a practical filter arrangement for consumer receivers which can be adjusted to very rapid-changing reception conditions.